Video signal processing apparatus which process signals in sampled data format, typically sample the video signal at instants determined by a sampling clock signal, which desirably bears a fixed relationship to the horizontal synchronizing signals. In addition, to facilitate processing of the signal the clock signal desirably will have a frequency which is a multiple of the chrominance subcarrier frequency. If the multiple is an even number, e.g., 4, for standard signals, there will be an integer number of cycles per horizontal interval. If a fixed relationship is maintained between the sampling clock and the horizontal synchronizing signal, corresponding sampling points in successive lines will be vertically aligned. Conversely, if this relationship is altered, corresponding sampling intervals in successive lines will not be vertically aligned. If the sampled signals are stored and processed before display, the non vertical alignment of sampling intervals (and thus of pixels) will be manifested in reproduced images by jagged vertical edges. This situation occurs, when for example, video signals are derived from video cassette recorders and video disc players, and the signals are processed in sampled data format to provide special effects such as freeze frame or picture-in-picture. This problem and a corresponding solution is discussed in detail in U.S. Pat. No. 4,630,098, entitled "Progressive Scan Display System With Correction For Non-Standard Signals". However the apparatus shown for calculating the sampling error or skew is relatively complicated, and requires a relatively long time period to calculate respective skew errors.